The present invention relates to multi-element antennas, and more particularly, to the switching of antenna elements for changing the radiation and receiving patterns of a radio frequency antenna.
For an airplane, it is desirable that the antenna pattern be changeable and have a band width of at least 25 percent with a cardioid pattern switchable to scan in 90.degree. steps. The simplest and most widely known method for producing a cardioid pattern is to place two dipoles or monopoles at a distance of one wavelength from each other and to feed the antenna elements 90.degree. out of phase. However, this presents difficulties in that for an airplane it is desirable that the antenna be flush mounted and a band width of 25 percent is difficult to obtain under all required circumstances. One approach to this problem is to use a flush mounted cavity which is exited either by monopoles or loops, with the number of loops depending upon how it is desirable to steer the beam. For example, if four loops are mounted in a cavity and fed with a linear phase delay of 360.degree. around the circumference, i.e. the four loops are exited with phases 0.degree., 90.degree., 180.degree., and 270.degree., the radiation pattern of the horizontal plane of the four loops will be omnidirectional as will the pattern of a monopole. However, while the phase of the radiated signal from the monopole will remain constant around the antenna in the horizontal plane, the phase of the signal from the four loops will change 360.degree.. Thus, for some direction, the signals from the four loops and the signal from the monopole will be in phase. If at the same time the ampitude of the two signals are equal, a null will be produced in the combined radiation pattern in the direction of the null. By changing the phase relationship in the plane of the monopole and the four loops, the beam can be steered in the horizontal plane.
Another variation is to excite only two oposing loops at 180.degree. out of phase. This approach gives a broader null and higher gain. However, whereas the four loops, can be easily scanned continuously in the horizontal plane, excitation of only two oposing loops permits radiation in 90.degree. steps.
It has been determined that for multi-element antennas the radiation pattern of the antennna is not as predicted. One reason for this is parasitic excitement between the elements and a second reason is that voltages induced between elements is not of the proper phase. For the antenna of the exemplary embodiment, it has been found that there is parasitic excitment between the two loops and that the voltages induced in the two loops from the central monopole was in phase rather than 180.degree. out of phase. In such a case, while the push-push mode signals in phase will never actually enter through the input terminal, they will excite the antenna in a push-push mode with respect to ground.
Accordingly, it is desirable to reduce the distortion radiation pattern of an antenna caused by parasitic excitation between the elements. It is also desirable to provide for the short circuiting of antenna elements when the voltages in the elements produce push-push currents in phase which are caused by parasitic excitation while letting the push-pull current (180.degree. out of phase) produced by an input voltage to pass unobstructed.